Biochar application to soil is currently being widely posited as a means to improve soil quality and thereby increase crop yield. Next to beneficial effects on soil nutrient availability and retention, biochar is assumed to improve soil water retention. However, evidence for such an effect in the primary literature remains elusive. Therefore, we studied the effect of biochar on soil hydrological characteristics in two separate field experiments on a sandy soil in The Netherlands. In Experiment I, biochar produced through slow pyrolysis of herbaceous feedstock at two temperatures (400 °C and 600 °C) was applied to soil at a rate of 10 t ha− 1. In Experiment II, the 400 °C biochar was applied at rates of 1, 5, 20 and 50 t ha− 1. Soils were analysed for soil water retention, aggregate stability and other soil physical parameters after three growing seasons and one growing season for Experiment I and Experiment II, respectively. We characterised the pore structure of the biochar using X-ray computed micro-tomography (XRT) and hydrophobicity using contact angle measurements.

We found no significant effects of biochar application on soil water retention in either experiment. Aggregate stability was also not significantly affected, nor was field saturated hydraulic conductivity. XRT analysis of the biochars showed that they were highly porous, with 48% and 57% porosity for the 400 °C and 600 °C biochar respectively. More than 99% of internal pores of the biochar particles were connected to the surface, suggesting a potential role for biochars in improving soil water retention. However, the biochars were highly hydrophobic. We postulate that this strong hydrophobicity prevented water from infiltrating into the biochar particles, prohibiting an effect on soil water retention. Our results suggest that, in addition to characterising pore space, biochars should be analysed for hydrophobicity when assessing their potential for improving soil physical properties.